Days with Minimum Temperature Above 90 Degrees

The urban heat island (UHI) is a phenomenon of higher nighttime temperatures in the urban core compared to the surrounding rural countryside. Urban surfaces cool more slowly because impervious surfaces trap heat during the day and then radiate that heat back into the atmosphere at night. Typical nighttime temperatures in urbanized Phoenix are 10°F warmer than surrounding rural areas. In addition, urban attributes such as transportation and industry elevate urban temperatures.

When temperatures do not drop below ninety degrees at night, nighttime temperatures provide less relief from the summer heat, air conditioners must run almost continually, increasing energy use, and investment opportunities for the region may be lost if people and companies choose to locate elsewhere.

Data Source:

Data were collected by Arizona State University's Decision Center for a Desert City (DCDC) using records from NOAA's National Climatic Data Center. Source: http://www.ncdc.noaa.gov/cdo-web/results; Phoenix Sky Harbor International Airport.

This graph shows the number of days that the low temperature was higher than ninety degrees for the years 1948 through 2013. Temperatures were recorded at the Phoenix Sky Harbor International Airport climate monitoring station.

Visualization Notes:

This graph shows the number of days that the low temperature, recorded at the Phoenix Sky Harbor International Airport climate monitoring station, was higher than ninety degrees for the years 1948 through 2013. Nighttime minimum temperatures above ninety degrees started occuring in the 1970s. This is an indication of the urban heat island phenomenon of higher nighttime temperatures in the urban core compared to the surrounding rural countryside. In each decade since, the number of days with a minimum temperature above ninety degrees has increased: from three in the 1970s, to six in the 1980s, to eight in the 1990s, to fifty in the most recent decade of 2000-2009. Although much of this increase has to do with the urban heat island effect, rising global temperatures may also contribute to warmer summer temperatures, resulting in higher nighttime lows.

Data Source

Data were collected by Arizona State University's Decision Center for a Desert City (DCDC) using records from NOAA's National Climatic Data Center. Source: http://www.ncdc.noaa.gov/cdo-web/results; Phoenix Sky Harbor International Airport.

This graph shows the number of days that the low temperature was higher than ninety degrees for the years 1948 through 2013. Temperatures were recorded at the Phoenix Sky Harbor International Airport climate monitoring station.